In high-voltage film resistors used for many types of applications, it is very important that there be a low voltage coefficient, it being understood that the voltage coefficient is the variation in resistance that occurs as applied voltage is increased or decreased (but not taking into account the resistance variation caused by self heating of the resistive film).
Another factor of major importance is the physical length of the resistor. The word "length" as here employed refers to the physical length of the substrate, as distinguished from the length of the resistive line on the substrate. It often occurs that the person designing a circuit board (or circuit assembly) can leave only a certain-length space for the resistor, and--furthermore--he or she normally wants the length of such space to be as short as possible. What is typically desired, therefore, is the highest possible voltage rating (voltage rating being the highest voltage that may be applied to the resistor) for a resistor that will fit in the shortest possible space.
To achieve the desired low voltage coefficient, the line of resistive film material is made long. Stated otherwise, the resistivity (ohms per square) of the film material is made lower while, at the same time, the line length is made greater. The longer line, with consequent reduced voltage gradient, results in the low voltage coefficient. However, the long line also results in small gaps between adjacent line sections. Accordingly, with a long line on a short substrate, and with high voltages being handled, there is the large risk that corona damage, resistor instability, and finally voltage breakdown (flashover), will occur.
Another important factor relative to many resistors is the inductance thereof. Often, the inductance must be made as low as possible, such as no greater than the inductance of a straight-line film the length of which is substantially the same as that of the substrate. There are, however, various applications where such minimized inductance (noninductance) is not demanded, where such factors as voltage coefficient, and substrate length, are more important.
Another major factor relative to many resistors, namely high-power resistors where maximum performance is to be achieved, is that migration of resistive film material can occur across gaps between apexes. Thus, for increased power and/or voltage ratings of power resistors, the tendency toward migration across the gaps must be reduced.
A crucial consideration is the cost of the resistor, and it is well known that the cost of capital equipment, and speed of production, are major factors regarding resistor cost. Screen-printed thick-film resistors have for decades proven to be relatively economical to produce. They are to be contrasted with (for example) resistors which require lasers or grinding machines for production in the desired patterns. It is much better to lay down (screen print) the desired film pattern initially, than to lay down a solid film and then--at major expense--laser-cut or grind it in order to form the pattern.
It is well known that cylindrical resistors, as distinguished from typical flat ones, are strong and relatively shock resistant, and have the capability of receiving long lines of film. Thus, for many uses, cylindrical film-type resistors are greatly desired.